Known heat exchangers generally comprise two spaced headers with a plurality of parallel, open ended tubes interconnecting the headers for fluid communication with each other. Serpentine fins are generally disposed between the spaced flat tubes.
The manufacture of such heat exchangers generally require holes or openings to be formed in the surface of each header to receive the open ends of the flat tubes. Typically, this is accomplished by a punching operation which forms the opening. Examples of such prior heat exchangers are seen in U.S. Pat. Nos. 4,098,331 and 4,825,941.
One of the problems encountered in the manufacture of such heat exchangers is that the thin walled header tubes are subject to circumferential pressure or stress during the punching operation causing the tube to collapse. In order to maintain the integrity of the header tube, it has been required to perform the punching operation while pressurizing the inside of the tube with pressurized fluid, or a solid core as taught in U.S. Pat. Nos. 3,487,668 and 5,052,478.
In addition, another problem with respect to such a method of manufacture is that raw unfinished edges of the slots left by the punching operation interfere with the insertion of the flat tubes.
Among the objectives of the invention are to provide a heat exchanger with improved headers and a method of making such headers; a heat exchanger with header tubes whose surface has been strengthened to withstand a punching operation to form slots therein without the need to pressurize the inside of the headers and to prevent collapse thereof during the punching operation; a header tube which facilitates insertion of a plurality of tubes simultaneously; and which assume a high quality, leak-free, braze bond between the flat tubes and the header tubes.
According to the invention, the header tubes are strengthened by deforming the surface thereof at longitudinally spaced locations to form spaced annular grooves with inclined sides and a base on the external surface of the groove and spaced annular ribs on the inner surface opposite the grooves. The tubes are deformed by rollers which form the spaced grooves and ribs along the length of the tube for added strength. The tubes are then punched to form slots in the base of the grooves.